Add global config variables for wordpress-multisite

Refactoring to allow global variables in wp-config (or your theme's functions.php). This way a wordpress multisites can configure the plugin for all blogs at once. The local/blog-specific configuration overwrites the global configuration. This works for the app_id and fallback_img at the moment since those are the two I needed and are likely to be used for multisites.

I added those lines to my wp-config:
```php
/** Set the multiside-wide config-variables for the http://wordpress.org/support/plugin/wp-facebook-open-graph-protocol plugin */
$wpfbogp_wpfbogp_app_id_global = '123123123';
$wpfbogp_fallback_img_global = 'http://www.betterplace.org/logo.png';
```